The present invention relates to the field of electronic circuitry, and more particularly to an improved signal processing circuit for optimizing speech recognition in potentially noisy ambient conditions. The predominant current usage of the present inventive adaptive speech filter is in the processing of signals from a microphone used for hands free telephonic communications, wherein it is desirable to produce a strong and clearly intelligible signal.
It is known in the art to amplify input audio signals, and to filter such inputs so as to accentuate the effective signal to noise ratio. It is also known in the art to combine such functions in an integrated circuit, or the like. An adaptive highpass filter manufacture by Gennum(trademark) Corporation is a high pass filter which is intended to accentuate speech over background noise, and which is designed and intended primarily for use in hearing aid devices, and the like.
There exists today a need for communications input devices, such as voice input microphones and similar type devices, which can be adapted to the ever increasing range of interpersonal voice communications means. Such means include, but are not limited to, conventional telephone devices, cellular and other portable telephone devices, long and short range radio communications devices, and the like. Although there have been numerous circuit designs, such as the Gennum(trademark) device discussed above, which are intended to reduce background noise under varying ambient conditions, none of the existing circuits has have been well adapted for use with such communications input devices. Among the desirable features for such an application which have not been adequately met in the prior art are a low operating current requirement, relative absence of a startup current surge, shared signal output and power input connections, adaptability to tailor input impedance and/or gain to a variety of microphone devices, and optimized filter response characteristics.
Clearly, it would be desirable to have a filter device with one or more of the above described desirable characteristics. However, to the inventor""s knowledge, no device has existed in the prior art which combines these characteristics, or even a significant subset thereof.
Accordingly, it is an object of the present invention to provide an adaptive speech filter which requires very low current to operate.
It is still another object of the present invention to provide an adaptive speech filter which effectively reduces background noise relative to intelligible speech frequency signal.
It is yet another object of the present invention to provide an adaptive speech filter which does not require an initial high startup current.
It is still another object of the present invention to provide an adaptive speech filter which has common signal output and power input lines.
It is yet another object of the present invention to provide an adaptive speech filter circuit which can be readily tailored to the sensitivity and frequency response of an input microphone.
It is still another object of the present invention to provide an adaptive speech filter which has filter response and timing characteristics well adapted to the requirements of a voice communication apparatus
It is yet another object of the present invention to provide an adaptive speech filter which will operate at a low voltage.
Briefly, a known embodiment of the present invention is an electrical circuit residing primarily on an application specific integrated circuit (xe2x80x9cASICxe2x80x9d). According to an embodiment of the inventive adaptive speech filter circuit, a preamp circuit, a highpass adaptive filter circuit, an output buffer circuit, a peak detector amplifier/filter circuit, a peak detector circuit and a voltage regulator circuit are provided. The preamp circuit provides frequency and gain compensation to match the inventive adaptive filter circuit for use with a variety of microphone input devices. The highpass adaptive filter circuit has a Chebyshev highpass filter for suppressing ambient low frequency noise, and the corner frequency varies according to the level of background noise. The output buffer circuit provides a low output impedance, high signal drive output. The peak detector amplifier/filter circuit is used to further condition the audio signal before it is sampled by the peak detector circuit. The peak detector circuit is a precision rectifier circuit used to detect the peak audio level, for modifying the corner filter frequency accordingly, and the voltage regulator circuit provides the reference and bias voltages to the other components of the adaptive filter circuit.
An advantage of the present invention is that background noise is effectively reduced while speech signal remains well intelligible.
A further advantage of the present invention is that little current is required to operate the device.
Yet another advantage of the present invention is that startup current is not significantly greater than normal operating current.
Still another advantage of the present invention is that the signal output and VCC power input are shared, thereby providing compatibility with convention telephonic communication lines.
Yet another advantage of the present invention is that the circuit can be readily configured for use with microphones having different sensitivities and frequency responses.
Still another advantage of the present invention is that peak detector timing is optimized for voice communications applications in relatively noisy environments.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of modes of carrying out the invention, and the industrial applicability thereof, as described herein and as illustrated in the several figures of the drawing. The objects and advantages listed are not an exhaustive list of all possible advantages of the invention. Moreover, it will be possible to practice the invention even where one or more of the intended objects and/or advantages might be absent or not required in the application.
Further, those skilled in the art will recognize that various embodiments of the present invention may achieve one or more, but not necessarily all, of the above described objects and advantages. Accordingly, the listed advantages are not essential elements of the present invention, and should not be construed as limitations.